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Hawking radiation - Wikipedia, the free encyclopedia Page 1 of 11 Hawking radiation From Wikipedia, the free encyclopedia Hawking radiation (also known as Bekenstein-Hawking radiation ) is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided the theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein [1] who predicted that black holes should have a finite, non-zero temperature and entropy. Hawking's work followed his visit to Moscow in 1973 where Soviet scientists Yakov Zeldovich and Alexander Starobinsky showed him that according to the quantum mechanical uncertainty principle, rotating black holes should create and emit particles. [2] The Hawking radiation process reduces the mass and the energy of the black hole and is therefore also known as black hole evaporation . Because Hawking radiation allows black holes to lose mass and energy, black holes that lose more matter than they gain through other means are expected to dissipate, shrink, and ultimately vanish. Smaller micro black holes (MBHs) are predicted to be larger net emitters of radiation than larger black holes; thus, they tend to shrink and dissipate faster. Hawking's analysis became the first convincing insight into a possible theory of quantum gravity. In September 2010, Hawking radiation was claimed to have been observed in a laboratory experiment, however the results remain unrepeated and debated. [3] Other projects have been launched to seek this radiation. In June 2008, NASA launched the GLAST satellite, which will search for the terminal gamma-ray flashes expected from evaporating primordial black holes. In speculative large extra dimension theories, CERN's Large Hadron Collider may be able to create micro black holes and observe their evaporation. [4][5][6][7][8] Contents 1 Overview 2 Trans-Planckian problem 3 Emission process 4 Black hole evaporation 5 Large extra dimensions 6 Deviation from Hawking radiation in loop quantum gravity 7 Experimental observation of Hawking radiation 8 See also 9 Notes 10 Further reading 11 External links Overview Black holes are sites of immense gravitational attraction. Classically, the gravitation is so powerful that nothing, not even electromagnetic radiation, can escape from the black hole. It is yet unknown how gravity can be incorporated into quantum mechanics, but nevertheless far from the black hole the gravitational effects can be weak enough for calculations to be reliably performed in the framework of quantum field theory in curved spacetime. Hawking showed that quantum effects allow black holes to emit exact black body radiation, which is the average thermal radiation emitted by an idealized thermal source known as a black body. The electromagnetic radiation is as if it were emitted by a black body with a temperature that is inversely proportional to the black hole's mass. Physical insight on the process may be gained by imagining that particle-antiparticle radiation is emitted from just beyond the event horizon . This radiation does not come directly from the black hole itself, but rather is a http://en.wikipedia.org/wiki/Hawking_radiation 5/24/2011 Hawking radiation - Wikipedia, the free encyclopedia Page 2 of 11 result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles. A slightly more precise, but still much simplified, view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). By this process, the black hole loses mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunneling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon. An important difference between the black hole radiation as computed by Hawking and thermal radiation emitted from a black body is that the latter is statistical in nature, and only its average satisfies what is known as Planck's law of black body radiation, while the former fits the data better. Thus thermal radiation contains information about the body that emitted it, while Hawking radiation seems to contain no such information, and depends only on the mass, angular momentum, and charge of the black hole (the no-hair theorem). This leads to the black hole information paradox. However, according to the conjectured gauge-gravity duality (also known as the AdS/CFT correspondence), black holes in certain cases (and perhaps in general) are equivalent to solutions of quantum field theory at a non-zero temperature. This means that no information loss is expected in black holes (since no such loss exists in the quantum field theory), and the radiation emitted by a black hole is probably the usual thermal radiation. If this is correct, then Hawking's original calculation should be corrected, though it is not known how (see below). As an example, a black hole of one solar mass has a temperature of only 60 nanokelvin; in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5 × 10 22 kg (about the mass of the Moon) would be in equilibrium at 2.7 kelvin, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb, and thereby lose mass. Trans-Planckian problem The trans-Planckian problem is the observation that Hawking's original calculation requires talking about quantum particles in which the wavelength becomes shorter than the Planck length near the black hole's horizon. It is due to the peculiar behavior near a gravitational horizon where time stops as measured from far away. A particle emitted from a black hole with a finite frequency, if traced back to the horizon, must have had an infinite frequency there and a trans-Planckian wavelength. The Unruh effect and the Hawking effect both talk about field modes in the superficially stationary space- time that change frequency relative to other coordinates which are regular across the horizon. This is necessarily so, since to stay outside a horizon requires acceleration which constantly Doppler shifts the modes. An outgoing Hawking radiated photon, if the mode is traced back in time, has a frequency which diverges from that which it has at great distance, as it gets closer to the horizon, which requires the wavelength of the photon to "scrunch up" infinitely at the horizon of the black hole. In a maximally extended external Schwarzschild solution, that photon's frequency only stays regular if the mode is extended back into the past region where no observer can go. That region doesn't seem to be observable and is physically suspect, so Hawking used a black hole solution without a past region which forms at a finite time in the past. In that case, the source of all the outgoing photons can be identified–it is a microscopic point right at the moment that the black hole first formed. The quantum fluctuations at that tiny point, in Hawking's original calculation, contain all the outgoing http://en.wikipedia.org/wiki/Hawking_radiation 5/24/2011 Hawking radiation - Wikipedia, the free encyclopedia Page 3 of 11 radiation. The modes that eventually contain the outgoing radiation at long times are redshifted by such a huge amount by their long sojourn next to the event horizon, that they start off as modes with a wavelength much shorter than the Planck length. Since the laws of physics at such short distances are unknown, some find Hawking's original calculation unconvincing. [9][10][11][12][13][14] The trans-Planckian problem is nowadays mostly considered a mathematical artifact of horizon calculations. [12][15] The same effect occurs for regular matter falling onto a white hole solution. Matter which falls on the white hole accumulates on it, but has no future region into which it can go. Tracing the future of this matter, it is compressed onto the final singular endpoint of the white hole evolution, into a trans-Planckian region. The reason for these types of divergences is that modes which end at the horizon from the point of view of outside coordinates are singular in frequency there. The only way to determine what happens classically is to extend in some other coordinates that cross the horizon. There exist alternative physical pictures which give the Hawking radiation in which the trans-Planckian problem is addressed. The key point is that similar trans-Planckian problems occur when the modes occupied with Unruh radiation are traced back in time. [16] In the Unruh effect, the magnitude of the temperature can be calculated from ordinary Minkowski field theory, and is not controversial. Emission process Hawking radiation is required by the Unruh effect and the equivalence principle applied to black hole horizons. Close to the event horizon of a black hole, a local observer must accelerate to keep from falling in. An accelerating observer sees a thermal bath of particles that pop out of the local acceleration horizon, turn around, and free-fall back in. The condition of local thermal equilibrium implies that the consistent extension of this local thermal bath has a finite temperature at infinity, which implies that some of these particles emitted by the horizon are not reabsorbed and become outgoing Hawking radiation. [16] A Schwarzschild black hole has a metric The black hole is the background spacetime for a quantum field theory.